Long-term Changes Of Surface Ozone And Its Impact Factors Over Northern China

Tropospheric ozone?O3?is a critical trace gas playing important roles in human health,air quality and ecosystem productivity,and it is the third most important greenhouse gas.Therefore,the research of the long-term changes of O3 and its influencing factors is helpful to understand the impact of human activities and meteorological factors on the atmospheric environment and their internal relations,and to develop more effective control strategies.Nitrogen oxide?NOx?and volatile organic compounds?VOCs?are two primary precursors of O3.The emissions of NOx have been decreasing since 201 1.while there is an increasing trend in VOCs emissions.Therefore.it is necessary to conduct a detailed study on the O3 changes over China during the past decade,and formulate more targeted prevention and control measuresIn order to understand the long-term changes of ozone and its impact factors over North China Plain?NCP?,we analyzed O3 pollution and trends during 2003-2015 based on the observed O3 at Mt.Tai,a typical high altitude site in NCP.Then we discussed the reasons of O3 changes by using varieties methods,including the back trajectory model?HYSPLIT?.satellite retrieved formaldehyde?HCHO?and NO2 column,GFDL-AM3 model,MODIS fire point data and ozone generation efficiency?OPE?.Results show that summertime O3 pollution was the most severe in the whole year,with high O3 levels both at day and night.The Nonctural baseline O3 concontration were 75-85 ppbv in June and 60-70 ppbv in July-August.The high concentrations of O3 at Mt.Tai were dominated by the strong photochemical process and high concentrations of regional baseline O3.We found that Mt.Tai is one of the most severely O3 polluted region in the mid-latitude of the North Hemisphere,with higher O3 concentration than those of most rural sites in NCP.A linear regression analysis shows that summertime O3 measured at Mt.Tai has increased significantly by 1.7±1.0 ppbv yr-1 for June and 1.8±1.0 ppbv yr-1 for July-August average.Both nocturnal regional baseline O3 and daily maximum 8 hour O3?MDA8 O3?showed an increasing trend,and the growth rate of MDA8 O3 was higher.The observed increase is supported by a global chemistry-climate model hindcast?GFDL-AM3?.Generally,Mt.Tai is one of the regions with high ozone increasing rate in the world.Subsequently,we analyzed the influencing factors of O3 at Mt.Tai by using variety method.The results showed that the O3 increase was mainly due to the increased emissions of its precursors,in particular VOCs.An important finding is that the emissions of NOx have diminished since 2011,but the increase of VOCs appears to have enhanced the OPE and contributed to the observed O3 increase in central eastern China?CEC?.We further quantified the influence of meteorological factors and precursor emissions on O3 variations over CEC using GEOS-Chem chemistry transport model and analyzed the budget of O3.The simulated monthly mean MDA8 O3 in July increased by approximately 13.6%,from 65.5±7.9 ppbv?2003?to 74.4±8.7 ppbv?2015?.The increase in averaged O3 in CEC resulting from the emission increase is 4.0±1.9 ppbv,which is higher than that caused by meteorological changes?3.1±4.9?ppbv relative to the 2003 standard simulation.The simulated average effect of emission changes is larger than the effect of meteorological changes,but the regions with larger O3 increases shows a higher sensitivity to meteorology than to emission changes.However,emission changes still dominate O3 changes in the polluted regions.Sensitivity tests indicate that increased levels of anthropogenic VOCs dominate the O3 increase over the eastern part of CEC,and anthropogenic NOx mainly increase MDA8 O3 over the central and western parts and decrease O3 in a few urban areas in the eastern part.In addition,the MDA8 O3 increased significantly.The emissions changes contribute about 71%of increasing 03,and meteorology contributes about 29%.Further more,the emissions of NMVOCs contributes about 50%for emission contribution,and NOx contributes about 31%.Thus the increase of emissions dominates the MDA8 O3 increase at Mt.Tai.Budget analysis showed that net photochemical production and meteorological conditions?transport in particular?are two important factors that influence O3 levels over CEC.Furthermore,we analyzed O3 pollutantion characteristics using data from China National Environmental Monitoring Center Network?CNEMC?during 2015-2018,and then calculated the ozone production ability of cities using TCEQ method.In addition.we also research on the impact factors of O3 precursors and meteorology and quantified the contributions of meteorology.We found that NCP in warm season?April-September?is the most severe O3 polluted region in China,.The O3 levels over NCP increased significantly during 2015-2018;the number of days exceeding standard II also increased.In addition,exceeded periods of standard has expanded.and the number of exceedance is more common in spring.We found that the O3 pollution during May-August was more severe than in the warm season,with MDA8 O3 increasing from 59.1±12.2 ppbv?2015?to 73.8±15.4 ppbv?2018?.The increase of MDA8 O3 was the most pronounced in June,increasing from 64.0±11.3 ppbv?2015?to 88.8±11.1 ppbv?201 8?.The TCEQ results showed that the regional MDA8 O3 increased from 48 ppbv?2015?to 67?ppbv??201 8?,and the local contribution?O3 formation ability in one city?decreased from 29 ppbv?2015?to 19 ppbv?2018?.The decrease of NO2 and PM2 5 and the slow increase of VOCs levels are important factors for O3 increases over NCP.The results of the reanalysis of meteorological data by MERRA-2 showed that temperature.as well as radiation are important factors for the O3 increase.The GEOS-Chem results showed that meteorological conditions led about 4.84±2.27 ppbv MDA8 O3 over NCP increase,accounting for about 20%of the actual growth.O3 increase in some areas?such as Shanxi?was larger than 10 ppbvIn summary,we found that the O₃ levels at Mt.Tai.which reflects the whole NCR,increased significantly during 2003-2018.with precursors and PM2.5 as the main influencing factors.However,the impact of meteorology has large spatial viriations over CEC.The results indicate that more strategies are needed to control ozone pollution.We should focus on controlling both VOC and NOx emissions.In addition,we should consider the feedback mechanism of PM2 5 concentration on O3 pollution.Transport issues in local O3 control strategies should go beyond transport from neighboring areas and account for the long-range transport.We suggest that the ability to predict extreme weather conditions should be improved,and the weather situation in the polluted areas should be evaluated according to the historical meteorological conditions,so as to strengthen our efforts on the polluted areas with high occurrence of adverse weather conditions in advance.This study is helpful for the scientific understanding of the roles of human activities and meteorological factors of air pollution in China,and provide a theoretical support for future O3 pollution control.